1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary battery. More specifically, this invention relates to a lithium secondary battery or other type of non-aqueous electrolyte secondary battery able to be used in a power source to be mounted in a vehicle.
2. Description of Related Art
Non-aqueous electrolyte secondary batteries such as lithium secondary batteries can be advantageously used as so-called portable power sources for personal computers, hand-held devices and the like or as power sources for vehicle propulsion. In particular, lithium ion secondary batteries able to achieve high energy density and low weight are becoming increasingly important as high output power sources for propelling vehicles such as electric vehicles (EV) and hybrid vehicles (HV). Among such non-aqueous electrolyte secondary batteries, those types of battery that use positive electrode active substances having a prescribed hollow structure can achieve high output with low state of charge (SOC), and are therefore preferred in applications where output at low SOC is required (for example, as power sources for vehicles such as HVs, plug-in hybrid vehicles (PHV) and EVs).
In addition, this type of secondary battery is typically constructed by housing an electrode body, which is obtained by laminating a positive electrode and a negative electrode via a separator, in a case together with a non-aqueous electrolyte. The above-mentioned separator has the function of electrically insulating the positive electrode from the negative electrode and also has the function of holding the non-aqueous electrolyte. Furthermore, the separator also has the function of softening when the battery generates heat and reaches a certain temperature (typically the softening point or melting point of the material that constitutes the separator), thereby blocking the conduction path of charge carriers (a shut down function). Some such separators are provided with a heat-resistant layer that contains a filler such as alumina in order to, for example, inhibit short circuiting caused by thermal contraction of the separator. In general, in cases where a battery generates heat due to, for example, a short circuit occurring inside the battery, the area around the point of short circuit on the surface of the negative electrode reaches a higher temperature than other parts of the battery. As a result, the above-mentioned heat-resistant layer is disposed so as to face the negative electrode. This type of heat-resistant layer is disclosed in Japanese Patent Application Publication No. 2011-253684 (JP 2011-253684 A).